The present application generally relates to the field of electric motor controls, and specifically to an automotive power steering control system.
Typically, automobiles today use power steering systems, such as electric power steering (EPS) systems. In an EPS system, a driver's effort to steer the automotive are assisted by powered systems driven electrically. For example, the EPS system provides a steering torque assist directly by an electric motor drive. The motor itself can include a three-phase permanent magnet synchronous motor, or any other motor. The motor is driven from power sources such as a vehicle battery through application of an inverter system. The drive is typically monitored and controlled by a processing unit with sensors to detect the operating conditions of the motor. The EPS drive assist is called into use based on driver demand. Specifically, as a driver applies a steering force in one direction, the EPS system supplies a further torque in the same direction. The EPS system provides damping for the road feel and rack/column effects for return and other features. In other words, the EPS system works in all four torque speed quadrants. However, in case of inverter failure, the torque from the EPS motor can generate steering torque as current conducts through shorted circuit elements and body diodes as the motor back EMF increases due to motor velocity, temporarily shorting the phases. For example, a driver could be turning left while the motor drive erroneously provides damping torque resulting in higher efforts, which is undesirable.
The motor may be operated by a current mode controller. For current mode control, the current flowing into motor terminals is measured and compared to a calculated reference current signal and are representative of desired current for the motor to affect desired operating conditions.
However, by using such conventional techniques relatively weaker shorts, which cannot be detected based on transistor drain to source voltage, go undetected. The weaker shorts in the semiconductor elements of the inverter circuit lead to current measurement errors and mutual coupling in the motor windings, which in turn may be reacted to in current mode loop before detection can be accomplished and changed to feed forward torque control, thus causing the current feedback to be wrong. This will create effort above manual conditions, which is undesirable. Further, the above techniques fail to distinguish an open semiconductor from a shorted semiconductor. An open semiconductor also leads undesirable effects in the driver assistance provided by the EPS system but need to be separately diagnosed because an open semiconductor will not cause shoot through and the inverter may still be commanded normally.
Accordingly, it is desirable to detect inverter errors, and identify root cause of the errors more precisely than conventional techniques to improve the operation of the EPS systems.